JERKING OR BUCKING WHILE DRIVING

Ford F-150

I own a 99 F-150 with 114,000 miles. Like some other questions I was scanning through, this just may be repetitive. I acquired my truck in the summer of 2004. Around November of 2005--may not have noticed the November of 2004--it began jerking during normal driving. Like others, a lot of money was spent to diagnose and correct the problem. The jerking stopped. Jumping forward to November 2006. It is happening again. Putting two and two together, it only happens during the cold weather months and when going up hill or during a strain. I live in Alabama, so only a couple of months go by with cold weather. Could this be something to do with the vacuum system or master cylinder? On that, when should I feel resistance when I apply my break pedal? Right now the pedal moves around 6 to 7 inches before I feel pressure. I can pump the pedal and it gets tighter. Thanks in advance for the help.

*Need little more informations about what was diagnose and what was corrected when the jerking stopped.
Technically thinking, there may be something else behind, even further the problem that was resolve causing the same problem to happen again.
As long as the PROBLEM(S) behind the problem is not solve, the problem will always come back.

The problem was never solved. Ford service could not find any thing. Told me to drive it until it got worse. The problem went away on its on. Now the cold weather is back and so is the problem. I don't think that it is a coincidence.

Sorry for not mentioning that earlier. But from some of the questions others are experiencing the same thing. What does it take for a recall to be announced?